A terminal station and relay station within a synchronous optical communication network include a device that detects an abnormality, such as a loss of signal, by monitoring the input level of an optical signal received from an opposite station or by calculating the error rate of received data. When an abnormality, such as an LOS (Loss of Signal) is detected, the terminal station and the relay station change over a communication line from a work system to a protection system to maintain a normal communication state, and output an alarm to an operator terminal within the station and to the next-stage terminal station.
FIG. 1 shows one configuration example of a conventional loss-of-signal detecting device in an optical transmitting device.
In FIG. 1, an optical signal transmitted from the opposite station is input to an optical transceiver 1 of the same station. Data obtained by O/E (Optical/Electrical) conversion is input to a CDR (Clock & Data Recovery) unit 2.
The optical transceiver 1 includes an optical-output constant control unit or the like (not shown). Upon detecting a signal equal to or smaller than a predetermined optical input level using an optical AGC control signal or the like, the optical transceiver 1 outputs a loss-of-signal (LOS) alarm corresponding to a severe fault (SF: Signal Failure) to a data processing unit 3.
The clock and data recovery unit 2 extracts a clock component contained in the input data signal, reproduces a received clock, and samples the input data using the extracted clock, thereby reproducing the data received from the opposite station. The data and clock reproduced by the clock and data recovery unit 2 are output to the data processing unit 3.
The data processing unit 3 decodes the received data to restore the data of the transmission origin. In this case, the data processing unit 3 calculates the bit error rate (BER) of received data, and determines the degradation level of the communication line. When the BER becomes equal to or higher than a predetermined value, the data processing unit 3 determines that a signal degradation (SD) corresponding to a milder fault than the SF occurs, and records this information.
Upon detecting a LOS alarm, the data processing unit 3 sequentially outputs data of continuous data values “0” to the next stage, to execute a change over from a work system to a protection system and to carry out the alarm process of line abnormality.
FIG. 2 shows another configuration example of a conventional loss-of-signal detecting device.
In the present example, in place of the LOS alarm from the optical transceiver 1 shown in FIG. 1, a loss-of-lock (LOL) alarm showing an asynchronous (self run) state output from a PLL (Phase Locked Loop) circuit or the like inside the clock and data recovery unit 2 is output to the data processing unit 3, by relating this alarm to the signal failure (SF). Other configurations are similar to those shown in FIG. 1. In the present example, a commercially available CDR device that satisfies the common specification of the MSA (Multi Source Agreement) as a private entity related to the SDH is used in the clock and data recovery unit 2.
In the present example, upon detecting the LOL alarm, the data processing unit 3 sequentially outputs data of continuous data values “0” to the next stage, to execute a change over from a work system to a protection system and to carry out the alarm process of line abnormality (see Patent Document 2).
(Patent Document 1) Japanese Patent Application Laid-open Publication No. 2001-339347
(Patent Document 2) Japanese Patent Application Laid-open Publication No. H7-95156
According to the conventional configuration shown in FIG. 1, the optical transceiver 1 outputs an LOS alarm based on the received optical level, regardless of a signal error (BER) calculated by the data processing unit 3. Therefore, there is a risk that faults may not be detected in the order of a mild fault SD and a serious fault SF. Usually, the LOS and SF in FIG. 1 are set to an equivalent fault level, and the SD is set to about BER IE-6(10−6), and the LOS (SF) is set to about BER IE-4(10−4).
As a result, there is a risk that a moderate fault of the SD in which the current system can continuously operate may be detected as a serious line fault SF. While maintenance staff of the station and vendors analyze the BER in the process up to the occurrence of the fault or at the time of the occurrence of the fault to search the cause of the fault, the staff and vendors cannot analyze the cause of the fault, and thus the fault monitoring using the BER does not operate effectively.
The conventional configuration shown in FIG. 2 utilizes the fact that a commercial CDR device 2 outputs the LOL (LOS of Look) alarm in the serious fault (SF) of about BER IE-4. However, the detection speed of the LOL, that is, the time from the occurrence of a loss of signal until the detection of an asynchronism cannot satisfy the SF detection time within the 100 μS prescribed by the specification GR-253 due to a constraint of the circuit configuration of a PLL and the like.
Therefore, in the Prior Art by utilizing the fact that the conventional optical transceiver 1 outputs a continuous signal of a data value “0” when an optical input level becomes equal to or lower than a predetermined level, the clock and data recovery unit 2 outputs the LOL alarm immediately after detecting this output of the continuous signal.
However, the optical transceiver 1 in recent years incorporates an amplifier relevant to the optical output constant control in order, to improve reception sensitivity and expand reception range. Therefore, even when the optical signal includes a small amount of noise level, the optical transceiver 1 outputs data containing amplified noise. Consequently, the clock and data recovery unit 2 at the next stage cannot use the above conventional method (detection of continuous zero).